Study On Molecular Aggregation And Photo-physical Properties Of Bi-1,3,4-oxadiazol And Bi-1,3,4-thiadiazole Derivatives

The past decade has witnessed a spurt of interest in the study of optical andelectronic properties of π-conjugated materials stimulated mainly by the potentialfor their application in devices such as light-emitting diodes, photovoltaic devices,and field effect transistors. Many of these devices, which make use of organicπ-conjugated polymers and small molecules as key elements, are rapidlyapproaching commercialization. An attractive feature of organic materials is theability to fine-tune their optical and electronic properties by subtle manipulation oftheir basic molecular structure. In this respect, although structure propertyrelationships of organic electronic materials are fairly well established at themolecular level, factors that control their bulk properties are still not fullyunderstood. Bulk properties such as luminescence, exciton migration, and carriermobility depend strongly upon the intermolecular dipole coupling, which aredetermined by the relative positions of adjacent molecules and directions of theirdipole moments. Understanding the nature of the interactions that determine thepacking of molecules in the solid state and how they affect the optical and electronicproperties of the materials is therefore essential for tuning their properties.Despite there are extensive studies on the emission properties in the solid stateof various organic compounds, however, our fundamental knowledge of solid-statefluorescence of organic molecules is rather limited and the understanding of thestructure-property relationship is still insufficient. Here, a detailed study on thephotophysical properties of bi-1,3,4-oxadiazole and bi-1,3,4thiadiazole derivatives, e.g.5,5’-bis（4-tetradecyloxyphenyl）-2,2’–bi-1,3,4-oxadiazole/thiadiazole,PP-OXD-14and BTD-14in solution and in the solid state providing a molecularlevel understanding of the factors controlling their solid-state luminescence behavioris reported. And the high symmetry of the molecular structure would make it anideal model for theoretical calculation. The combination of theoretical calculationsand spectroscopic studies on the molecular self-assembled structures has shed lighton the interplay between intrinsic luminescent properties and molecular packing inthe solid state.The obtained results are outlined as follows:Strong blue fluorescent emissions were observed in PP-OXD-14either insolutions or in solid state. In nonpolar solvents such as cyclohexane(1×10^-5mol/L),fluorescence quantum yields Ф_f（69%） for PP-OXD-14was considerably high, andin polar solvent，such as THF, Ф_f（34%） decreased. The lower Фfin the polarsolvents can be attributed to the highly efficient non-radiative decay from the ICTexcited state.The aggregation behavior of PP-OXD-14in THF could be explained well bythe changes of quantum yields on concentration. The initial decrease of quantumyields could be attributed to concentration-quenching or the formation ofH-aggregates, and the consequent increase is probably caused by the enlargingpopulation of J-aggregates in the concentrated solutions. All monomer, H-aggregatesand J-aggregates in solution could be reserved in the drop-cast films, and thepresence of J-aggregates and the energy transfer path from H-aggregates toJ-aggregates were considered to contribute to the relative high solid statefluorescence quantum yield （50%）, and with heat-treated to solid film, the quantumyield still present a high sequent, about56%, which implied a stable structure in thesolid of PP-OXD-14.Similar to PP-OXD-14, BTD-14exhibited the same aggregation behavior insolutions. H-and J-aggregates could be formed simultaneously in chloroformsolutions of BTD-14with moderate concentration (10^-4mol/L), and the populationof J-aggregates enlarges during further concentration increase. All monomers, H-aggregates and J-aggregates in solutions could be reserved in the drop-cast films,and the presence of J-aggregates and the energy transfer path from H-aggregates toJ-aggregates were considered to contribute to the relative high solid statefluorescence quantum yield （33%）. The BTD-1dimer potential energy surface （PES）was computed with M062x/6-31G**method, and the molecular packing patterncorresponding to the lowest minimum of the PES are in good agreement with thecrystal structures. Exploring the efect of molecular packing on its electronicstructure with the TD-M062x method revealed that J-aggregates could be formed byenlarging the intermolecular displacement along the molecular long axis by about9.8A.In a word, these investigations revealed that the molecular aggregation could beturned by concentration or temperature, providing a new idea to control themolecular self-assembled structure, and thus the electronic and optical properties.